Architecture critical to dependability
Auxiliary power systems are as important to iPMCCs’ dependability as the main power system and communication architecture.
iPMCC stands for intelligent power and motor control centre. iPMCCs are low-voltage power and motor control switchboards that incorporate intelligent motor protection relays. They are widely used in industrial sites and large facilities where processes are continuous and downtime critical.
We can identify three types of network architectures in an iPMCC:
- The main electrical power system that supplies the loads.
- Communication and automation network data.
- Auxiliary power supply network which supplies power to multiple electronic devices that are internal to the switchboard.
It is my contention that where continuity of service is at a premium, the three architectures’ levels of dependability should be aligned. Of particular importance is that the auxiliary power supply architecture should be coherent with the dependability of the communications architecture.
The critical place of the auxiliary power system in dependability can be overlooked. It is sometimes considered just a design feature.
What methodology is used to determine dependability coherence?
The methodology I use may be summarised like this:
- Determine the level of dependability “The Customer Entity” requires of an iPMCC for its processes. (“The Customer Entity” stands for customers and specifications in market segments worldwide with continuous, critical processes.)
- Define the dependability levels of the 3 iPMCC architectures and perform dependability assessments for each architecture.
- Present the coherent dependability combinations of the 3 architectures.
What are the customer dependability requirements?
I have broken down into three groups the main customer dependability requirements:
- No requirement
- Partial redundancy – but still with single points of failure (SPOFs)
- Full redundancy – no SPOF.
A frequently unexpressed but mandatory operational constraint in continuous processes is the need for withdrawable MCC functional units which can be pulled out in the event of a fault without impacting switchboard operation.
How many dependability levels are there?
For keep things short and sweet, I have defined three levels of dependability. They are expressed in terms of failure rates. Dependability level 3, for example, shows 1 failure every 100 years.
What is the general dependability rule governing auxiliary power?
The table below shows coherent dependability combinations for the three systems.
Of course the most coherent combination is that all three architectures should have the same levels of dependability.
The general rule that emerges, however, is: for dependability to be acceptable, the dependability of the auxiliary power system should be equal to or greater than that of the communications architecture.
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